Sara Stearns

Document Type

Honors Thesis


Extensive variation in the size and structure of the vertebrate brain is well documented. How the architecture of the brain evolves in response to divergent selection remains an open question. One theory states brain structures evolve in concert; a competing theory states components evolve independently in a mosaic pattern. Tests within a single species are nonexistent. A killifish (Rivulus hartii) is found in localities that differ in the presence and absence of fish predators on the island of Trinidad. We showed previously that decreased predation is associated with evolution of a larger overall brain size in male Rivulus. Here we tested whether the architecture of the brain evolves in concert or as a mosaic in response to divergent predatory selection. We compared 2nd generation lab reared specimens from sites with and without predators for differences in size of the telencephalon, optic tectum, cerebellum, and dorsal medulla. We observed genetically-based shifts in the architecture of the male (but not female) brain. Male Rivulus from sites that lack predators exhibited significantly larger brain regions when compared with fish from sites with predators. All structures were also significantly positively correlated. Our results support the concerted pattern of evolution.

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